Date: Wed, 10 Dec 2014 09:25:27 -0500 From: Steve Grubb <sgrubb@...hat.com> To: oss-security@...ts.openwall.com Subject: Re: Offset2lib: bypassing full ASLR on 64bit Linux On Tuesday, December 09, 2014 10:09:02 PM Steve Grubb wrote: > On Tuesday, December 09, 2014 08:03:10 PM Daniel Micay wrote: > > On 09/12/14 11:18 AM, Steve Grubb wrote: > > > 4) Then I started wondering about the heap when you use other memory > > > manager libraries such as jemalloc. This turned out to be interesting. > > > You get about 19 bits of randomness using it. Its not as bad as non-PIE > > > glibc but not as good as PIE glibc. You also got the same amount of > > > randomness whether the app was PIE or not. This is an area ripe for more > > > experimenting, exploiting, and patching. Supposedly some of these heap > > > managers use mmap as the underlying allocator. So, why aren't they > > > getting 29 bits, too? :-) > > > > Your measurement of the difference is quite accurate. > > There's other allocators, too. > > libtalloc: > $ ./all-bits > heap 14 bits > pie-heap 29 bits > > Hoard: > $ ./all-bits > heap 25 bits > pie-heap 25 bits tcmalloc: $ ./all-bits heap 11 bits pie-heap 26 bits and just so they are all in one place: jemalloc: $ ./all-bits heap 19 bits pie-heap 19 bits glibc: $ ./all-bits heap 14 bits pie-heap 29 bits Are there any other allocators in common use? This is quite a range in heap ASLR just based on which library you link against. Might be nice if some of the low performers gain some more bits of randomness. -Steve > Different allocators, different strategies, different randomness. While > people are thinking about this, it might be a good time to check everything > that's popular. Hmmm...now that I think about it, I haven't looked for > address bias in the last samples.... :-) > > -Steve > > > The page multiple constraint zaps 12 potential bits of entropy, but > > jemalloc's 4M chunk alignment increases that to 22 bits. I'm not sure > > what can be done about it because there's a very strong performance case > > for the design. > > > > I sent in a fix for the MALLOC_CONF part of this at least, so an > > attacker won't be able to reduce it further: > > > > https://github.com/jemalloc/jemalloc/pull/174 Download attachment "signature.asc" of type "application/pgp-signature" (182 bytes)
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